Measurements of free radicals over a period of 4.5 years in gamma-irradiated ultra-high molecular weight polyethylene

Gamma irradiation-induced free radicals have been investigated for 4.5 years in medical grade ultra-high molecular weight polyethylene resins, GUR 4120, GUR 4150 and Himont 1900, at room temperature (23 deg. C) in absence (vacuum) or presence of oxygen (air), and at 75 deg. C in vacuum. In vacuum, the primary radicals (predominantly allyl) decayed following Y=YR+ae-bt and, in 30 days at 75 deg. C or 100 days at 23 deg. C, their concentration was reduced from ∼1017 radical/g (initial) to a minimum YR∼1015 radical/g and remained at this level as long as the samples were kept in vacuum. When the samples were exposed to air, at any time during this study, these residual radicals were found to decay to a very stable, polyenyle-type terminal radical